0:04Skip to 0 minutes and 4 secondsI am here again at the University of Tokyo with Professor Yasunobu Nakamura, an expert on superconducting qubit. Professor Nakamura, tell us about devices you are building. Okay, we are trying to build a small quantum information processing system using superconducting qubits. So there we put several superconducting qubits as an array and then control them to manipulate the quantum state to demonstrate simple quantum information processing. How do you execute one and two qubit gates using superconducting system?

0:40Skip to 0 minutes and 40 secondsYes, so as you know, it is very important to apply very precise control on superconducting qubit and for that we are using microwave pulses typically in a timescale of 10 nanosecond to 100 nanosecond to implement quantum gate for single qubit control and 2 qubit gate. How do you make your superconducting chip? Yes, similarly to silicon technology, we use lithography for making superconducting devices, so like optical lithography and electron beam lithography are used to create superconducting chips in a tiny scale. Okay, what is unique about your approach?

1:28Skip to 1 minute and 28 secondsOkay, so now we would like to put superconducting qubits in array in a two-dimensional way, so qubits will be arranged on a small chip in a two-dimensional array and then we would like to control the quantum state to implement simple quantum information processing. Okay. Tell us what's the challenge for building large scale quantum system using your technique.

1:58Skip to 1 minute and 58 secondsOkay, the important thing is that we need to control many qubit, so for that we need many wiring to control and read out the signal and that is quite challenging because chip – on the chip qubits are densely aligned and then there are not much – there is not much space left for the wiring, so the arranging all the wiring properly and also sending and read out the signal properly is quite challenging. That’s our near term target. Okay, thank you professor Nakamura.

Superconducting systems

In an earlier Step, we visited the laboratory of Professor Yasunobu
Nakamura at the University of Tokyo, and learned about using electric
charge and magnetic flux as our qubit state variable. In this video,
we continue that visit and learn more about how those experiments are
conducted.

Experiments in a refrigerator

The large can behind Professor Nakamura in the video is a dilution
refrigerator, which has several stages that are each progressively
cooler. Inside the dil fridge is a large rig that passes through the
stages, like this:

The figure below shows how a qubit is controlled from the outside (as
we saw in the video), with different parts of the circuit at different
temperatures: room temperature (“RT” in the figure), 4 Kelvin, then 10
millkelvin – 1/100th of a degree above absolute zero! (Quantum dot
experiments use a similar physical setup to cool their chips.)

At the bottom stage is a block of aluminum with two halves, each with
a single connector for a coaxial cable. Mounted inside is a small,
rectangular chip. This one has a single transmon qubit that is about
one millimeter in size, as you can see in the figure below. (The
Josephson junction itself is small.)

Future designs

The figure below is a computer rendering of a new chip design. The
qubits are the yellow concentric circles at each lattice site. The
inner and outer rings are bridged with a Josephson junction to form a
transmon qubit. The qubit has four arms to interact with its
neighbors. The qubits are controlled from below the chip, as you can
see in the cutaway.

The four meander lines are readout resonators, used to measure the
state of a qubit. Four qubits share one readout connection which is
connected to a coaxial cable beneath the chip (not shown).

The small white holes that pass through the chip connect the red
sheets on the top and bottom, called ground planes, to stabilize the
voltage used as ground. The yellow control lines from the bottom are
coaxial cables, shown in cutaway, used to execute one-qubit rotations
and couple two neighboring qubits, so that we can perform two-qubit
gates like CNOT.

The picture is just a schematic and not the final design, but it gives
you the idea of what it takes to create a system, rather than just a
single qubit in an experiment. If this design is successful,
Professor Nakamura’s team will be able to build chips with tens or
hundreds of qubits on them. Designs like this may be able to run
surface code error correction, which we will introduce in the next
Activity.

Superconducting qubits, especially transmons, are large size compared
to transistors. We won’t be able to put many qubits into a single
chip, and entangling two superconducting qubits in separate chips is
one of the major outstanding challenges.